A traditional problem in the prior art has been identifying when a fluid reservoir is in an empty or near empty condition. For example, numerous devices, including the device, for example, described in my U.S. Pat. No. 4,137,915, have been developed to control flow rate and to identify the existence of an empty or near-empty condition. If a reservoir has emptied without the knowledge of the medical personnel attending the patient, a number of undesirable effects may result. The medical risks resulting from the absence of flow from the intravenous needle inserted into the patient make common the practice of inserting a new intravenous needle into the patient if additional fluid from a new reservoir is to be introduced to the patient. The introduction of a succession of needles to a patient causes trauma to the patient, and requires extra attention of medical personnel to the patient.
A common prior art approach to this problem has been the use of fluid control devices that include a "keep vein open" ("KVO") mode of operation. In this mode, which is sometimes designated as an "alarm" state, fluid is permitted to enter the patient only at a substantially slower rate than under normal conditions. In this manner, the rate at which the reservoir is emptied is substantially reduced, so as to prolong the period of time before flow stops. Such control devices, however, are typically bulky, expensive, and complicated. Their complexity makes them prone to failure and misuse. Moreover, such devices often require external sources of power.
There is disclosed, in U.S. Pat. No. 4,173,222, issued for an invention of Muetterties, a device that causes flow to revert to a KVO flow rate when the reservoir has emptied. That device, however, involves the use of two drip chambers and the use of systems for eliminating air flow through a passage when the KVO mode has been entered. Consequently, the successful operation of that device is conditioned in part on its success in blocking air flow when it enters the KVO mode.